1. Field of the Invention
The present invention relates to an apparatus and a method of forming images on both sides of a recording medium.
2. Discussion of the Background
Image forming apparatuses, such as copying machines, printers and facsimile machines, are known to include devices that are configured to form images on both sides of a recording medium (hereinafter sometime referred to as a sheet). Such image forming apparatuses capable of recording images on both sides of a sheet generally transfer an image of one side of an original, which has been formed and visualized on an image bearing member, onto one side of a sheet, and then fix the image onto the sheet by a fixing device. The sheet is then reversed by a reversing path etc., and is conveyed again so that an image of the other side of the original, which has been also visualized on the image bearing member, is transferred and fixed onto the other side of the sheet.
In the above image forming apparatuses, because a sheet conveying direction has to be reversed, and a sheet tends to be curled when an image is fixed onto one side of the sheet, reliability of sheet conveyance is hard to obtain. Further, a curled sheet causes inferior transfer of a toner image, resulting in deteriorating the image quality. Japanese Patent Laid-open Publications No. 1-209470, No. 3-253881 and 10-142869 respectively disclose an image forming apparatus in which toner images, which have been transferred onto both sides of a sheet from a first image bearing member and a second image bearing member, are fixed at one time.
Japanese Patent Laid-opened Publication No. 1-209470 discloses an image forming apparatus that transfers a first image formed on a photoconductor onto a transfer belt by a first transfer device, and then transfers a second image formed on the photoconductor onto one side of a sheet by a second transfer. The image forming apparatus then transfers the first image on the transfer belt onto the other side of the sheet by a second transfer device, thus transferring the images on both sides of the sheet. The sheet is then conveyed to a fixing device, where the images are fixed onto both sides of the sheet.
Japanese Patent Laid-open Publication No. 3-253881 discloses another image forming apparatus, which is similar to the one disclosed in the above-described JP Publication No. 1-209470. In the image forming apparatus, polarity of a second image formed on a photoconductor, which has been visualized as a toner image, is reversed on the photoconductor before a transfer process so that transfer of toner images onto both sides of a sheet is enabled without requiring a second transfer device. The sheet is then conveyed to a fixing device where the toner images on both sides of the sheet are fixed thereupon respectively.
Japanese Patent Laid-open Publication No. 10-142869 discloses another image forming apparatus that includes two transfer devices. After transferring color images onto both sides of a sheet, the sheet is conveyed to a fixing device, and the images are fixed onto both sides of the sheet at one time at the fixing device. The image forming apparatus includes a spur having a plurality of protrusions on its circumferential surface so as to guide conveyance of a sheet carrying unfixed toner images on both sides thereof.
Each of the above-described image forming apparatuses separates a sheet, carrying unfixed toner images on both sides thereof, from a transfer belt to convey the sheet to a fixing device. Therefore, the toner images carried on both sides of the sheet tend to be disturbed when the sheet is being conveyed separated from the transfer belt or when the sheet abuts on the fixing device.
Further, each of the image forming apparatuses disclosed in JP Publications No. 1-209470 and No. 3-253881 does not include a guide device to guide a sheet so as to be conveyed to a fixing device. Therefore, the sheet is not smoothly conveyed to the fixing device and an image carried on the sheet is disturbed or the sheet is jammed thereby reducing reliability for the image forming apparatus. Furthermore, it is relatively difficult to make the speeds of a transfer belt and a fixing device substantially the same. Therefore, when a sheet being conveyed by the transfer belt reaches a fixing device which has a conveying force generally larger than that of the transfer belt, the sheet starts to be conveyed by the fixing device at the speed of the fixing device. Thereby, images carried on the sheet are easily disturbed, causing image blurring as a result.
In the image forming apparatus of JP Publication No. 10-142869, an unfixed image tends to touch the spur, thus easily causing deterioration of an image quality. Further, as described above, because it is relatively difficult to make the speeds of a transfer belt and a fixing device substantially the same, when a sheet being conveyed by the transfer belt reaches a fixing device which has a conveying force generally larger than that of the transfer belt, the sheet is conveyed by the fixing device at the speed of the fixing device thereby causing images carried on the sheet to be easily disturbed and, as a result, image blurring.
FIGS. 1(a)-1(d) are schematic drawings for explaining background processes of transferring toner images onto both sides of a sheet. In FIG. 1(a) illustrating a state of a developing and a first transferring processes, a negatively charged toner image of a first side of an original formed on a photoconductor drum 1 serving as a first image bearing member is transferred onto an intermediate transfer belt 10 serving as a second image bearing member 10 by a first transfer device 21 applying a positive voltage. In FIG. 1(b) illustrating a state of a second developing process, another negatively charged toner image of the second side of the original is formed on the photoconductor drum 1, and at the same time the first side toner image carried on the intermediate transfer belt 10 reaches a position where the first side toner image has been transferred onto the intermediate transfer belt 10 from the photoconductor drum 10 after making one round. Further, a sheet is conveyed so as to be correctly positioned relative to the first and second side toner images.
In FIG. 1(c) illustrating a state of a second transferring process, the negatively charged second side toner image on the photoconductor drum 1 is transferred onto the second side of the sheet by the first transfer device 21 applying a positive voltage. At this time, the first side toner image on the intermediate transfer belt 10 is overlapped with the sheet. In FIG. 1(d) illustrating a third transferring process, a second transfer device 22 is turned on to apply a positive voltage, so that the negatively charged first side toner image on the intermediate transfer belt 10 is transferred onto the first side of the sheet. The sheet is then conveyed to a fixing device (not shown) and a cleaning operation is performed for the intermediate transfer belt 10.
Thus, the transfer process is performed three times. In particular, in the third transferring process, the first side toner image on the intermediate transfer belt 10 is transferred onto the sheet by applying the voltage with the second transfer device 22 from the side of the second side of the sheet onto which the second side toner image has been transferred from the photoconductor drum 1. Therefore, because of an effect of charging, the second side toner image transferred on the second side of the sheet tends to be disturbed. Further, increase of the charge of the sheet may cause electrostatic offsetting of the toner image in the fixing operation.
Japanese Patent Laid-open Publication No. 3-253881 discloses an image forming apparatus in which transfer of images onto both sides of a sheet is realized by a transfer device arranged at the side of the first side of the sheet. The first image formed on a photoconductor is transferred onto a transfer belt by a first transfer belt, and the polarity of a second image formed on the photoconductor is reversed on the photoconductor before a transfer process is performed, so that the polarities of the first and second images differ from each other. Thereby, the first and second images are transferred onto both sides of a sheets at the same time by a single transfer device.
Japanese Patent Laid-open publication No. 2000-105513 discloses use of two developing devices to differentiate the polarities of first and second images. Further, the polarity of a transfer voltage is made changeable, so that the first and second image can be transferred onto both sides of a sheet at the same time.
FIGS. 2(a)-2(d) are schematic drawings for explaining processes of transferring images onto both sides of a sheet in the image forming apparatus of JP publication No. 3-253881. In FIG. 2(a) illustrating a state of developing and a first transferring processes, a negatively charged toner image formed on the photoconductor drum 1 is transferred onto the intermediate transfer belt 10 by the transfer device 21 applying a positive charge. In FIG. 2(b) illustrating a state of a second developing process, the polarity of another negatively charged toner image formed on the photoconductor drum 1 is reversed to a positive charge by charge of a corona charger 6. In FIG. 2(c) illustrating a state of a second transferring process, the voltage applied by the transfer device 21 is switched to a negative charge, so that the first and second images are transferred onto both sides of a sheet at the same time. In FIG. 2(d) illustrating a state of a conveying process, the sheet is conveyed to a fixing device and a cleaning operation is performed for the intermediate transfer belt 10.
Further, an image forming apparatus of JP publication No. 2000-105513 includes two developing devices containing developer having different polarities. Further, the polarity of a voltage to be applied to a transfer roller, provided inside of a loop of an intermediate transfer belt at a position where contacting each other, is changeable. When transferring a first image (positively charged) from a photoconductor to the intermediate transfer belt (first transferring), a transfer voltage of a negative polarity is applied to the transfer roller. Then, the transfer voltage is changed to a positive polarity, so that the first image (positively charged) is transferred onto the first side of a sheet and at the same time the second image (negatively charged) is transferred from the photoconductor onto the second side of the sheet (second transferring).
In each of the above image forming apparatuses, although the charging polarities of toner images are made different from each other, the polarity of a transfer voltage must be switched between the first transferring and the second transferring, which complicates the mechanism and the control of transfer voltage switching.
The present invention has been made in view of the above-discussed and other problems, and addresses the above-discussed and other problems.
Preferred embodiments of the present invention provide a novel image forming apparatus and a novel image forming method that fix toner images transferred onto both sides of a sheet without disturbing the images, thereby avoiding deterioration of the images. The preferred embodiments of the present invention further provide a novel image forming apparatus and a novel image forming method that transfer images onto both sides of a sheet with a relatively simple mechanism and a relatively simple control and that avoid disturbing the images transferred onto the sheet. The preferred embodiments of the present invention further provide a novel image forming apparatus and a novel image forming method using a novel heat-resisting transfer belt that enables stable transfer, sheet conveyance and fixing operations so that satisfactory image quality can be obtained.
According to a preferred embodiment of the present invention, an image forming apparatus includes a first image bearing member, a second image bearing member, and a fixing device. A first visual image formed on the first image bearing member is transferred onto the second image bearing member to be transferred from the second image bearing member onto a first side of a recording medium, and a second visual image formed on the first bearing member is transferred from the first image bearing member onto a second side of the recording medium, so that the visual images are obtained on the first and second sides of the recording medium respectively. The visual images on the first and second sides of the recording medium are fixed by the fixing device in a state that the second image bearing member and the recording medium are overlapped with each other.
According to another preferred embodiment of the present invention, an image forming apparatus includes a first image bearing member, a second image bearing member, a fixing device, a first transfer device configured to transfer a first visual image carried by the first image bearing member onto the second image bearing member or a second visual image carried by the first image bearing member onto a recording medium, and a second transfer device configured to transfer the first visual image carried by the second image bearing member onto the recording medium. The first visual image is transferred from the second image bearing member onto a first side of the recording medium and the second visual image is transferred from the first image bearing member onto a second side of the recording medium, so that the visual images are obtained on the first and second sides of the recording medium respectively, and the visual images on the first and second sides of the recording medium are fixed by the fixing device in a state that the second image bearing member and the recording medium are overlapped with each other.
According to another preferred embodiment of the present invention, an image forming apparatus of electrophotography, comprising includes a first image bearing member having a photoconductive property, a developing device configured to visualize an image on the first image bearing member into a toner image, a second image bearing member configured to carry thereupon the toner image transferred from the first image bearing member; and a fixing device. A first toner image formed on the first image bearing member is transferred onto the second image bearing member to be transferred from the second image bearing member onto a first side of a recording medium, and a second toner image formed on the first image bearing member is transferred onto a second side of the recording medium, so that the toner images are obtained on the first and second sides of the recording medium respectively. The toner images on the first and second sides of the recording medium are fixed by the fixing device in a state that the second image bearing member and the recording medium are overlapped with each other.
The above image forming apparatuses may further include a charging device, and after the first visual image formed on the first image bearing member is transferred from the first image bearing member onto the second image bearing member, a charging polarity of the first visual image is reversed on the second image bearing member by the charging device, so that the first and the second visual images are transferred onto the first and second sides of the recording medium at a same time.
In the above image forming apparatuses, the second image bearing member may be shaped in a form of an endless belt, and the endless belt second image bearing member may have a heat-resisting property against heat of 150-300xc2x0 C. and a volume resistivity of 106-1012xcexa9xc2x7cm, and may be formed in two layers including a substrate member and a surface layer.
According to still another preferred embodiment of the present invention, an image forming apparatus includes a first image bearing member, a second image bearing member, a charging device, and a fixing device. A first image formed on the first image bearing member is transferred onto the second image bearing member, a second image is formed on the first image bearing member, the first and second images are transferred onto both sides of a recording medium at a same time, and the recording medium is conveyed to a fixing area of the fixing device by the second image bearing member, and after the first image is transferred from the first image bearing member onto the second image bearing member, a charging polarity of the first image is reversed on the second image bearing member by the charging device so that the first and the second images are transferred onto the both sides of the recording medium at the same time.
According to still another preferred embodiment of the present invention, an image forming apparatus includes a first image bearing member, and a second image bearing member. A first visual image transferred from the first image bearing member onto the second image bearing member is transferred onto a first side of a recording medium and a second visual image is transferred from the first image bearing member onto a second side of the recording medium, so that the visual images are formed on the first and second sides of the recording medium. The second image bearing member includes a heat-resisting transfer belt having a heat-resisting property against heat of 150-300xc2x0 C. and a volume resistivity of 106-1012xcexa9xc2x7cm and formed in two layers including a substrate member and a surface layer.
According to another preferred embodiment of the present invention, a transfer belt for use in an image forming apparatus has a heat-resisting property against heat of 150-300xc2x0 C. In the apparatus, a first visual image transferred from a first image bearing member onto a second image bearing member is transferred onto a first side of a recording medium from the second image bearing member and a second visual image is transferred from the first image bearing member onto a second side of the recording medium, so that the visual images are formed on the first and second sides of the recording medium. The transfer belt is formed in two layers including a substrate member and a surface layer, and has a volume resistivity of 106-1012xcexa9xc2x7cm
According to still another preferred embodiment of the present invention, an image forming method includes steps of transferring a first image formed on a first image bearing member onto a second image bearing member, transferring a second image formed on the first image bearing member onto a second side of a recording medium, transferring the first image from the second image bearing member to a first side of the recording medium, and fixing the first and second images on the first and second sides of the recording medium in a state that the second image bearing member and the recording medium are overlapped with each other.
The above method may further include a step of reversing a charging polarity of the first image on the second image bearing member. In this case, the transferring of the second image formed on the first image bearing member onto the second side of a recording medium and the transferring of the first image from the second image bearing member to the first side of the recording medium are performed at a same time.
According to still another preferred embodiment of the present invention, an image forming method includes steps of transferring a first image formed on a first image bearing member onto a second image bearing member, reversing a polarity of the first image on the second image bearing member, transferring the first image on the second image bearing member onto a first side of a recording medium and a second image formed on the first image bearing member onto a second side of the recording medium at a same time, and conveying the recording medium to a fixing area of a fixing device by the second image bearing member.